Preview
IB DP Biology — Unit D: Continuity and Change | SL | D2.2 Gene Expression
📋 What You Will Learn
- Identify the mechanisms by which gene expression is regulated at the transcriptional level, including the role of transcription factors and promoter regions
- Explain how epigenetic modifications — DNA methylation and histone modification — alter gene expression without changing the DNA sequence
- Analyse how environmental factors can influence epigenetic marks and gene expression patterns
- Explain the concept of cell differentiation and how identical genomes can produce functionally distinct cell types through differential gene expression
- Apply understanding of gene regulation to contexts including development, disease, and inheritance of epigenetic marks
🔍 Guiding Question How can cells with identical DNA sequences develop into hundreds of distinct cell types, and how do environmental signals leave lasting marks on the genome without altering the genetic code itself?
🌍 Why This Matters Identical twins share the same DNA sequence, yet as they age they can develop strikingly different diseases, body compositions, and even personalities. The explanation lies not in their genes but in how those genes are switched on and off — a molecular conversation between genome and environment that continues throughout life.
The Problem Every Cell Must Solve
Every cell in your body — from a rod photoreceptor in your retina to an insulin-secreting beta cell in your pancreas — carries the same 3.2 billion base pairs of DNA. Yet these cells look nothing alike, behave nothing alike, and produce entirely different sets of proteins. This is not a paradox. It is the central achievement of multicellular life: the ability to use one genome as the instruction manual for thousands of different cell types, reading only the relevant chapters at the right time.
The mechanism that makes this possible is gene regulation — the molecular control of which genes are transcribed, when, and at what rate. Understanding gene regulation means understanding how a fertilised egg becomes an organism, how a cell responds to stress, and how errors in this system can lead to cancer. It is, in a very real sense, the operating system of life.
📖 Key Definition — Gene Expression Gene expression is the process by which the information encoded in a gene is used to synthesise a functional gene product — typically a protein, via transcription and translation, or a functional RNA molecule.
📖 Key Definition — Gene Regulation Gene regulation refers to the mechanisms that control whether a gene is expressed, the rate of its expression, and the timing and location of that expression within an organism.
Transcriptional Control: The First Gate
The most energy-efficient point to regulate a gene is before transcription even begins. If RNA polymerase never binds, no mRNA is made, no ribosome is recruited, and no protein is synthesised. The cell saves enormous resources by controlling the very first step. This is transcriptional regulation — and it is the dominant form of gene control in eukaryotes.
Promoters and the Transcription Start Site
Every gene has a promoter — a specific DNA sequence upstream (5') of the coding region where RNA polymerase and its associated proteins must bind before transcription can begin. In eukaryotes, RNA polymerase II cannot bind the promoter alone. It requires a set of general transcription factors that assemble at the promoter to form the pre-initiation complex. Without this assembly, the gene is silent.
📖 Key Definition — Promoter A promoter is a regulatory DNA sequence located upstream of a gene's coding region to which